The first detector in a television signal receiver converts radio-frequency (RF) signals in a selected one of the television broadcast channels, which channels occupy various 6-MHz-wide portions of the electromagnetic wave frequency spectrum, to an intermediate-frequency (IF) signals in one particular 6-MHz-wide portion of that spectrum. This conversion is typically carried out by superheterodyning the RF signals, which is to say mixing the RF signals with local oscillations from a first local oscillator oscillating at a frequency substantially higher than the frequencies in the television channel of highest frequency, which mixing is done by linear multiplication in a first mixer. The first mixer is preferably of doubly balanced type. The first detector is used to convert a selected RF signal to IF signal in order that up to 60 dB or more amplification can be done in that particular 6-MHz-wide portion of that spectrum using IF amplifiers with fixed, rather than variable, tuning. Amplification of the received signals is necessary to raise them to power levels required for further signal detection operations, such as video detection and sound detection in the case of analog TV signals, and such as symbol decoding in the case of digital TV signals. The first detector usually includes variable tuning elements in the form of preselection filter circuitry for the RF signals to select among the various 6-MHz-wide television channels and in the further form of elements for determining the frequency of the local oscillations used for super-heterodyning the RF signals.
In analog TV signal receivers, which generally employ single-conversion radio receivers, the frequency of the oscillations supplied by the first local oscillator is often fine-tuned in response to an electric fine-tuning signal. This fine-tuning signal is generated by an automatic fine-tuning (AFT) feedback loop, which includes a bandpass filter responsive to the video carrier component of an IF amplifier response and an AFT detector that generates the electric fine-tuning signal. The AFT detector typically includes a limiter amplifier and a frequency discriminator tuned for prescribed video carrier frequency as translated to IF by the first detector. AFT is done to adjust the IF signal, so that the frequency-modulated (FM) audio carrier component of the selected TV channel as it appears in the amplified IF signal supplied to the video detector falls into the in-channel sound trap filter, and so that the FM audio carrier of an adjacent TV channel next below in frequency as it appears in the amplified IF signal supplied to the video detector falls into the adjacent-channel sound trap filter. Also, the IF amplifier amplitude response is rolled off 6 dB at video carrier frequency to provide matched filtering for the vestigial sideband (VSB) filter in the analog TV transmitter, so the IF signal should be aligned so this amplitude equalization is correctly performed.
In some analog TV receivers of recent design first local oscillator signals are generated using a frequency synthesizer in which the first local oscillator signals are generated with frequency regulated in adjustable offset from the fixed frequency of a standard oscillator. This is advantageous if the frequency of the first local oscillations is to be fine-tuned in response to an electric fine-tuning signal, since electric fine-tuning of the frequency of the standard oscillator can be done (e. g., by using a varactor diode in an LC tank circuit) with the sensitivity of absolute frequency adjustment of first local oscillations to electric fine-tuning signal being constant no matter what the nominal frequency of the first local oscillator signals is.
DTV receivers known in the prior art have not used AFT of the first local oscillator. The data carrier of the VSB signal is located 310 kHz from the lower limit frequency of the 6 MHz TV broadcast channel, and the uppermost sideband extends to 310 kHz from the lower limit frequency of the 6 MHz TV broadcast channel. The IF amplifier bandwidths have been 6 MHz wide with less than a 1 dB ripple in amplitude response across the passband, so critical (fine) tuning has not been required. However, the inventor points out, the introduction of in-channel and adjacent-channel sound trap filtering into the IF amplifier chain of an DTV receiver is advantageous in reducing the artifacts of NTSC signal interference that will, during a transition era in which DTV broadcasting is done while analog TV broadcasting continues still to be done, accompany symbol codes recovered in a DTV receiver. If such sound trap filtering is introduced into the IF amplifier chain amplifying DTV signals, it becomes advantageous to use AFT of the first local oscillator in a DTV receiver during the reception of DTV signals.
Television signal receivers capable of receiving both digital television (DTV) signals transmitted in accordance with the ATSC broadcast standard and analog TV signals transmitted in accordance with the NTSC broadcast standard are described in the inventor's U.S. patent application Ser. Nos. 08/825,711, 08/820,193, filed Mar. 19, 1997 and respectively entitled RADIO RECEIVER DETECTING DIGITAL AND ANALOG TELEVISION RADIO-FREQUENCY SIGNALS WITH SINGLE FIRST DETECTOR and DIGITAL-AND-ANALOG-TV-SIGNAL RECEIVERS, EACH WITH SINGLE FIRST DETECTOR AND SHARED HIGH-BAND I-F AMPLIFICATION. These applications are incorporated herein by reference for providing details of receiver construction not directly related to developing and utilizing automatic fine-tuning signals. The TV receivers described in these applications are of plural-conversion type, each using a single first detector both DTV signals and analog TV signals. The first detector generates ultra-high-frequency (UHF) intermediate-frequency signals. There are differing requirements for IF amplification of DTV signals and IF amplification of analog TV signals, so each of these TV receivers uses different IF amplifier chains for analog TV signals and for DTV signals. In some of these TV receivers the different IF amplifier chains for analog TV signals and for DTV signals do share some IF amplifier stages, however. The first detector generates ultra-high-frequency (UHF) intermediate-frequency signals which are subjected to frequency-selective filtering with bandwidth(s) just sufficient to pass the broadcast signal that is currently selected for reception, so automatic fine tuning (AFT) of first local oscillator signal is a practical necessity. When a single first detector is used both for DTV signal reception and for analog TV signal reception, problems of how properly to develop AFT signals arise.
The derivation of AFT signals from the response of the IF amplifier chain for the amplitude-modulated NTSC video carrier is known from experience in analog TV signal receiver design to have problems, which problems arise owing to the 6 dB rolloff of video carrier that is customary for match filtering the vestigial sideband filter at the transmitter and at the same time helping in the design of the adjacent channel sound trapping. Accordingly, in an analog TV signal receiver design using a quasi-parallel IF amplifier chain for intercarrier sound, the bandpass filter for selecting video carrier to the AFT detector is connected to receive the response of the quasi-parallel IF amplifier chain, rather than the response of the IF amplifier chain for the amplitude-modulated NTSC video carrier supplied to the video detector.